An officer is a member of an armed force or uniformed service who holds a position of authority. Used without further detail, the term "officer" almost always refers to commissioned officers, the most senior portion of a force who derive authority from a commission from a state. In its broadest sense, the term "officer" also includes lower level leaders who are referred to as non-commissioned officers.

The proportion of officers varies greatly. Officers typically make up between an eighth and a fifth of modern armed forces personnel. In 2013, officers were the senior 17% of the British armed forces,[1] and the senior 13.7% of the French armed forces.[2] In 2012, officers made up about 18% of the German armed forces,[3] and about 17.2% of the United States armed forces.[4]

Historically, however, armed forces have generally had much lower proportions of officers. During the First World War, fewer than 5% of British soldiers were officers (partly because World War One junior officers suffered very high casualty rates, which were even higher in the British Army because officers' different headgear made them obvious targets). In the early twentieth century, the Spanish army had the highest proportion of officers of any European army, at 12.5%, which was considered unreasonably high by both foreign observers and many Spanish politicians.

Within a nation's armed forces, armies (which are larger) tend to have a lower proportion of officers, but a higher total number of officers, while navies and air forces have higher proportions of officers. For example, 13.9% of British army personnel and 22.2% of the RAF personnel were officers in 2013, but the army had a larger total number of officers.[5]

Having officers is one requirement for combatant status under the laws of war, though these officers need not have obtained an official commission or warrant. In such case, those persons holding offices of responsibility within the organization are deemed to be the officers, and the presence of these officers connotes a level of organization sufficient to designate a group as being combatant.

Commissioned officers are typically the only persons, in an armed forces environment, able to act as the commanding officer (according to the most technical definition of the word) of a military unit.[6] A superior officer is an officer with a higher rank than another officer, who is a subordinate officer relative to the superior.

Non-commissioned officers (NCOs) in positions of authority can be said to have control or charge rather than command per se (although the word "command" is often used unofficially to describe any use of authority).

In the United Kingdom, there are three routes of entry for British Armed Forces (BAF) officers. The first, and primary route are those who receive their commission directly into the officer grades following completion at their relevant military academy. In the second method, an individual may gain their commission after first enlisting and serving in the Junior Ranks, and typically reaching one of the Senior Non-Commissioned Officer (SNCO) ranks (which start at Sergeant (Sgt), and above), as what are known as 'Direct Entry' or DE officers (and are typically and informally known as an 'ex-ranker'). The third route is similar to the second, in that they convert from an enlisted to a commission; but these are only taken from the highest ranks of SNCOs, and are known as 'Late Entry' or LE officers. LE officers, whilst holding the same Queen's Commission, generally work in different roles from the DE officers. In the infantry, a number of Warrant Officer Class 1s (WO1) are commissioned as LE officers.

Newly commissioned U.S. Navy and Marine Corps officers celebrate their new positions by throwing their midshipmen covers into the air as part of the U.S. Naval Academy class of 2005 graduation and commissioning ceremony

A smaller number of officers may be commissioned via other programs, such as the Marine Corps Platoon Leaders Class (PLC) during summers while attending college, or the Navy's since discontinued Aviation Officer Candidate School (AOCS) program, which also included the embedded Aviation Reserve Office Candidate (AVROC) and Naval Aviation Cadet (NAVCAD) programs. Others may attend pre-commissioning or post-commissioning officer indoctrination programs for officers in the medical/dental specialities, lawyers slated as Judge Advocate General's Corps (JAG) officers, and military chaplains.

Graduates from the United States Service academies attend their institutions in an active duty status for no less than four years, and are commissioned immediately upon graduation; they comprise approximately 20% of the U.S. armed forces officer corps.

Other officers may be commissioned through the Reserve Officer Training Corps (ROTC), which is the largest source of officers for the U.S. Army and the U.S. Air Force. ROTC is composed of small training programs at several hundred American colleges and universities.[7] There is no Marine Corps ROTC program per se, but there exists a Marine Corps Option for selected midshipmen in the Naval ROTC program. The Coast Guard has no ROTC program.

An officer may also be commissioned at a federal or, for a limited number of Army National Guard (ANG) officers, at a state-based Officer Candidate School. These schools train and commission college graduates and enlisted personnel toward being promoted to the commissioned officer ranks.

In Commonwealth nations, commissioned officers are given commissioning scrolls (also known as commissioning scripts) signed by the Sovereign or the Governor General acting on the monarch's behalf. Upon receipt, this is an official legal document that binds the mentioned officer to the commitment stated on the scroll.

Non-commissioned members rise from the lowest ranks in most nations. Education standards for non-commissioned members are typically lower than for officers (with the exception of specialised military and highly-technical trades; such as aircraft, weapons or electronics engineers). Enlisted members only receive leadership training after they are promoted to positions of responsibility, or as a prerequisite for such. In the past (and in some countries today but to a lesser extent), non-commissioned members were almost exclusively conscripts, whereas officers were volunteers.

A non-commissioned officer (NCO) is an enlisted member of the armed forces holding a position of some degree of authority who has (usually) obtained it by advancement from within the non-commissioned ranks. Officers who are non-commissioned usually receive management and leadership training, but their function is to serve as supervisors within their area of trade specialty. Senior NCOs serve as advisers and leaders from the duty section level to the highest levels of the armed forces establishment, while lower NCO grades are not yet considered management specialists. The duties of an NCO can vary greatly in scope, so that an NCO in one country may hold almost no authority, while others such as the United States and the United Kingdom consider their NCOs to be "the backbone of the military" due to carrying out the orders of those officers appointed over them.[8]

In most maritime forces (navies and coast guards), the NCO ranks are called petty officers and chief petty officers, while enlisted ranks prior to attaining NCO/petty officer status typically known as seaman, or some derivation thereof. In most traditional infantry, marine and air forces, the NCO ranks are known as sergeants and corporals, with non-NCO enlisted ranks referred to as privates and airmen.

However, some countries use the term commission to describe the promotion of enlisted soldiers. Especially in countries with mandatory service in the armed forces. These countries refer to their NCOs as professional soldiers, rather than as officers.

In some branches of many armed forces, there exists a third grade of officer known as a warrant officer. In the armed forces of the United States, warrant officers are initially appointed by the Secretary of the service and then commissioned by the President of the United States upon promotion to chief warrant officer. In many other countries (as in the armed forces of the Commonwealth nations), warrant officers often fill the role of very senior non-commissioned officers. Their position is affirmed by warrant from the bureaucracy directing the force—for example, the position of regimental sergeant major in regiments of the British Army is held by a warrant officer appointed by the British government.

In the U.S. military, a warrant officer is a technically focused, single specialty officer, like a helicopterpilot or IT specialist; they are given salutes and they are addressed as "Mr", "Miss", "Sir", or "Ma'am". There are no warrant officers in the U.S. Air Force. While the ranks exist in the USAF, the last warrant officers retired in the 1980s. Each of the other U.S. Armed Forces have warrant officers—though each warrant accession program is unique to the individual service's needs. Despite Warrant Officers normally having more years in service than regular commissioned officer counterparts, their pay is slightly less than regular commissioned officers. In the United States armed forces, commissioned officers and, on certain instances, commissioned warrant officers are the only officers allowed to command units.

Officers in nearly every country of the world are segregated from the enlisted soldiers in many facets of military life. Facilities accommodating needs such as messing (i.e., mess hall or mess deck versus officers mess or wardroom), billeting/berthing, domiciles, and general recreation facilities (officer clubs versus NCO Clubs and CPO Clubs versus enlisted clubs) are separated between officers and enlisted personnel. This class system, historically correlated to socioeconomic status, is focused on discouraging fraternization and encouraging professional and ethical relations between officers and enlisted military personnel.[9]

Officers do not routinely perform physical labor; they typically supervise enlisted personnel, either directly or via non-commissioned officers. Commissioned officers will and do perform physical labor when operationally required to do so, e.g., in combat. However, it would be very unusual for an officer to perform physical labor in garrison, at home station or in homeport.